Method and arrangement in a communication network

ABSTRACT

The present invention relates to methods, a user equipment and a radio base station in a communication network, in which a downlink out-of-coverage is detected based on measurements done on a common channel or on the combination of common and dedicated channels. The out-of-coverage is then reported to the network, either using resources proactively assigned to the user equipment, or by transmitting a predetermined pattern of signature sequences assigned to the user equipment.

This application is a continuation of U.S. patent application No.12/598,264, filed Oct. 30, 2009, which is a National Stage of PCTApplication No. PCT/EP2008/003533, filed Apr. 30, 2018, which claimspriority from Swedish Patent Application Serial No. 0701053-1 filed May2, 2007, the disclosure of which is incorporated here by reference.

TECHNICAL FIELD

The present invention relates to methods and arrangements in acommunication network and more particularly to a downlinkout-of-coverage detection and reporting of the out-of-coverage to thenetwork.

BACKGROUND OF THE INVENTION

In E-UTRAN Orthogonal Frequency Division Multiple Access (OFDMA)technology is used in the downlink. OFDM is a modulation scheme in whichthe data to be transmitted is split into several sub-streams, where eachsub-stream is modulated on a separate sub-carrier. Hence in OFDMA basedsystems, the available bandwidth is sub-divided into several resourceblocks or units as defined, for example, in 3GPP TR 25.814: “PhysicalLayer Aspects for Evolved UTRA”. According to this document, a resourceblock is defined in both time and frequency. According to the currentassumptions, a resource block size is 180 KHz and 0.5 ms in frequencyand time domains, respectively. The overall uplink and downlinktransmission bandwidth is as large as 20 MHz.

Downlink Measurements in E-UTRAN

In E-UTRAN the user equipment (UE) is required to perform differenttypes of measurements in order to facilitate many radio resourcemanagement (RRM) related tasks such as scheduling, handover, admissioncontrol, congestion control etc. Some typical downlink measurementscarried out by the UE include channel quality indicator (CQI), carrierreceived signal strength indicator (carrier RSSI), reference signalreceived power (RSRP) etc. Some of these measurements are done on thereference symbols, which are transmitted at least once every 68sub-carrier in the frequency domain. For instance, RSRP and CQI aremeasured over the reference symbols. Other common channels sent on thedownlink in E-UTRAN include synchronization channel (SCH) and broadcastchannel (BCH).

Some of these measurements especially RSRP, which is based on long termaveraging, is used by the network for coverage triggered handover. Inother words RSRP could provide information related to the cell coverage.

Packet Oriented Transmission in E-UTRAN

The E-UTRAN is a packet oriented system, where all type of uplink anddownlink transmission including data and signalling takes place viashared channel. The network has the full control over the radio andnetwork resources or the so-called scheduling grants (e.g. resourceblocks, modulation, coding, sub-frames etc), which are assigned to theUE on demand basis. However, it might be possible, partly or fully, topre-assign limited amount of resources to the UE for certain type ofcritical periodic measurement reports.

RACH Transmission in E-UTRAN

Random access in E-UTRAN is based on a two-step procedure. In the firststep the UE transmits a randomly selected signature to the network. Inthe subsequent procedure the network responds to the UE with an uplinkscheduling grant which is used by the UE to transmit further detailsrelated to the connection request.

The transmitted sequence is randomly chosen out of an available pool of64 unique sequences. If the UE does not receive an uplink schedulinggrant within a certain time it randomly selects a new signature sequenceand performs a new random access (RA) attempt.

The time-frequency resources where random access may be performed areannounced via system information. One random access opportunity (orresource) is 1.08 MHz wide (6 resource blocks, each comprised of 180 kHzin the frequency domain) and lasts for 1 ms. Multiple RA opportunitiesmay be spread out over frequency. It is up to the network whether toschedule other data in a RA slot or not. The network thus also controlswhether RA transmission is orthogonal to shared data transmission ornot.

Out-of-Coverage Concept

An explicit out of coverage concept is not specified. However, similarconcept called out of sync is used in the WCDMA. In WCDMA the downlinkpower control is mandatory. This means the base station adjusts itsdownlink transmitted power in response to the power control commands(TPC) sent by the UE. In case of out of sync situation there is a riskthat excessive power up commands may saturate the base station downlinktransmitted power. Thus the main purpose of out of sync in WCDMA is toprotect the base station from transmitting unnecessary high power. Inother words when out of sync is reported the base station disables theUE connection or at least simply ignore the received TPC commands fromthe UE. The general concept of out of sync concept is described below.

The UE monitors downlink channel quality on a suitable measurementsignal (e.g. reference or pilot signals). If the estimated downlinkquality remains below an acceptable quality limit (Qout) over timeperiod (Tout) then the UE reports out-of-coverage to its higher layers(e.g. layer-3 or RRC) through an out-of-coverage primitive. SubsequentlyUE higher layers indicate to the RRC in the network (e.g. RNC) that theUE physical layer has detected out-of-coverage. The network then takesan appropriate measure such as change downlink power offsets, handover,congestion control etc.

Since the downlink radio condition improves, hence the UE also needs tomonitor downlink channel quality when in the state of out-of-coverage.In this situation if the UE detects that the estimated downlink qualityhas become greater than another threshold (Q_(in)) over time periodT_(in), then the UE reports in-coverage to its higher layers throughprimitive. Subsequently the UE higher layers inform the network that itsphysical layer has detected in-coverage.

Out of Sync Concept in WCDMA

As stated above no explicit out-of-coverage concept is specified.However, similar concept called out-of-sync reporting procedure is usedin WCDMA to protect the base station transmitted from transmittingunnecessary high power.

In WCDMA downlink out-of-sync is specified in the specification document3GPP 25.214 “Physical Layer Procedures” and is briefly described asfollows:

The UE estimates downlink channel quality, which is expressed astransmit power control (TPC) command error rate. If the downlink qualityis below specified level (Q_(out)) over T_(out) the UE reportsout-of-sync. Typically out of sync is reported to the network by the UEif the downlink measured quality in terms of TPC command error rateexceeds 30% over 160 ms measurement interval.

Since TPC commands are sent on DPCCH or F-DPCH, therefore the out ofsync criteria is based on the dedicated channel, i.e. on UE specificchannel. An overview of the out of sync procedure in WCDMA is depictedin FIG. 1.

Scenarios for Out of Coverage in E-UTRAN

In this section we described some important scenarios or situationswhere explicit coverage loss indication is needed by the network. Someexamples of such scenarios are:

-   -   E-UTRAN coverage border    -   Poor coverage at the junction of irregular cell sizes        Coverage Border Scenario:

Limited E-UTRAN coverage in some geographical vicinity may result incoverage boundaries. This may easily lead to a situation where a UEloses coverage when moving out of the E-UTRAN coverage borders. Anexplicit coverage loss Indication in this scenario would make thenetwork aware of the UE coverage status more clearly. This could helpthe network to improve the coverage, if possible, by employing suitableradio resource management technique. On the other hand, a consistentcoverage loss as depicted by the coverage loss criteria could allow thenetwork to drop such a UE well in time thereby saving network resources.

Poor Coverage Areas:

E-UTRAN like any other cellular network is expected to offer ubiquitouscoverage in all types of locations. However, there are at least someinevitable coverage spots, where good cell planning is very hard torealize in practice. These areas could be found at the intersection ofvery irregular cell sizes e.g. cells encompassing hilly and skyscraperswithin a congested metropolitan region. Consistently ensuring goodcoverage in such awkward coverage spots could drain enormous networkradio resources. However, an efficient radio resource managementmechanism could react and improve the coverage on demand basis byallocating more resources etc. The realization of such a mechanismrequires explicit UE feedback when its coverage falls below the desiredlevel.

Limitation of Implicit Coverage Loss Indication

The UE reported measurements such as RSRP in E-UTRAN may implicitlydepict the coverage status of the UE. However, any measurement includingRSRP is limited to a certain minimum reporting value, e.g. up to −140dBm. However, in such out of coverage scenarios the measured quantity(e.g. RSRP) is likely to be out of the reporting range, i.e. much lowerthan the minimum reportable value. In case the reported range is furtherextended (e.g. below −140 dBm), the measured results at lower rangeswould obviously incorporate very large measurement uncertainties. Thisis because at low measured quantity the uncertainty becomes largerrendering the reported value highly unreliable. Hence in thesecircumstances, solely based on the UE measurement reports, the networkmay not correctly infer that the UE is out of coverage or not.

Coverage Indication Via Dedicated Connection

In out of coverage or out of sync scenario the downlink connection (frombase station to UE) becomes unreliable. In WCDMA, where a dedicatedconnection is maintained, the UE is still able to send out of syncindication on the uplink despite the downlink remains unreachable. InE-UTRAN only shared channel is used for uplink and downlinktransmission, where resources are allocated by the network on demandbasis. It is thus unlikely that UE in E-UTRAN will be able to correctlyreceive any scheduling grant or resource allocation from the network inout of coverage situation. Thus the current WCDMA approach is lesslikely to work in E-UTRAN.

Coverage Loss Criteria Based on Dedicated Reference Signals

The existing out-of-sync procedure in UTRAN takes into account only theDPCCH or F-DPCH when estimating the downlink channel quality. As aconsequence the downlink channel quality estimate is done on dedicatedpilot bits and/or TPC commands.

However, it has been evaluated that downlink channel quality estimatebased on both dedicated and common pilots leads to better out-of-syncdetection by the UE. The possible reason is that dedicated referencesignals or pilots used in WCDMA are power controlled that does notprovide the actual coverage status of the UE. The aspect of using thecommon reference signals to detect out-of-sync or out-of-coverage is notused in the existing system.

SUMMARY OF THE INVENTION

In the preferred embodiment the downlink out of coverage detection isbased on measurement done on some common channels such as BCH, commonreference signals, synchronization signals or combinations thereof. Inanother embodiment it's based on the combination of any set of commonreference signals and dedicated reference signals.

The out of coverage is reported to the network in a number of ways. Itis indicated: By sending a unique pattern of signature sequences on RACHchannel.

By proactively allocating resources when certain UE reportedmeasurement(s) is at minimum level; or

By defining out of coverage indicator in one or more of the UEmeasurement reports.

Thanks to the provision of the inventive methods following advantageswill be obtained:

-   -   Explicit downlink out-of-coverage reporting would allow the        network to improve the coverage by taking appropriate action,        e.g. handover, congestion control etc.    -   On the other hand it would help network to save resources if        coverage cannot be improved.    -   It will improve network planning and dimensioning.    -   The UE is able to report out of coverage without the need for        requesting uplink scheduling grants or resource assignment.

Still other objects and features of the present invention will becomeapparent from the following detailed description considered inconjunction with the accompanying drawings. It is to be understood,however, that the drawings are designed solely for purposes ofillustration and not as a definition of the limits of the invention, forwhich reference should be made to the appended claims. It should befurther understood that the drawings are not necessarily drawn to scaleand that, unless otherwise indicated, they are merely intended toconceptually illustrate the structures and procedures described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, wherein like reference characters denote similarelements throughout the several views:

FIG. 1 shows an example of a communication network architecture;

FIG. 2: Out-of-coverage and in-coverage reporting principle;

FIG. 3: Out of coverage indication by unique pattern of signaturesequences in time.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 depicts a communication system, such as a OFDM system or a WCDMAsystem, including a Radio Access Network (RAN), such as the evolved UMTSTerrestrial Radio Access Network (E-UTRAN) architecture, comprising atleast one Radio Base Station (RBS) (or Node B) 15 a-b, connected to oneor more Radio Network Controllers (RNCs) 10 (only one shown in FIG. 1).The RAN is connected over an interface such as the Iu-interface to aCore network (CN) 12, which may be a connection-oriented external CNsuch as the Public Switched Telephone Network (PSTN) or the integratedServices Digital Network (ISDN), and/or a connectionless external CN asthe Internet.

The RAN and the CN 12 provide communication and control for a pluralityof user equipments (UE) 18 a-d. The UEs 18 each uses downlink (DL)channels (i.e. base-to-user or forward) and uplink (UL) channels (i.e.user-to-base or reverse) to communicate with at least one RBS 15 over aradio or air interface.

According to a preferred embodiment of the present invention, thecommunication system is herein described as a WCDMA communicationsystem. The skilled person, however, realizes that the inventive methodand arrangement works very well on all communications system. The userequipments 18 may be mobile stations such as mobile telephones(“cellular” telephones) and laptops with mobile termination and thus maybe, for example, portable, pocket, hand-held, computer-included orcar-mounted mobile devices which communicate voice and/or data with theRAN.

There are two main aspects of the invention:

Out of coverage detection

Out of coverage reporting

Out-of-Coverage Detection

The downlink out-of-coverage detection is either based on common channelor combination of common and dedicated channels. Hence the following twocategories of out-of-sync reporting procedures are discussed:

-   -   Detection based on common channel    -   Detection based on combined common and dedicated channels        Detection Based on Common Channel

In E-UTRAN some examples of common reference signals or channels are:

-   -   Downlink reference signals    -   Primary and/or secondary SCH channel    -   Primary BCH channel

The UE should consistently measure and evaluate the coverage losscriteria over a specified time period. For example the out of coveragecould occur when one or more of these quantities fall below a desiredthreshold over a time period (T_(1,out)):

-   -   Reference signal (RS) SINR or signal strength    -   SCH SINR or signal strength    -   Combined RS and SCH SINR or signal strength    -   BCH BLER    -   BER or symbol error rate

The UE would also report in coverage (i.e., when downlink coveragebecomes acceptable) when any one or more of the above measuredquantities would become greater than another threshold during certaintime period (T_(1,in)).

The above description is expressed in algorithmic form in the followingsections:

Out-of-Coverage Procedure:

UE reports out-of-coverage based on the following algorithm:

ρ=DL quality estimated by UE on common channel.

Start:

IF (ρ<ρ_(out)) over period T_(1,out)

UE reports out-of-coverage

ELSE

Go to Start.

In-Coverage Procedure:

The precondition for in-coverage reporting is that UE went toout-of-coverage state from in-coverage (based on common channel).

Start:

IF (ρ>ρ_(in)) over period T_(1,in)

UE reports in-coverage

ELSE

Go to Start.

Out of Coverage Based on Existing UE Measurements:

Since some UE reported neighbour cell measurements (e.g. RSRP) are basedon reference signals, therefore another possibility is to define out ofcoverage occurs when the measured quantity is below the minimumreportable value by certain threshold (Ψ). As an example the out ofcoverage will occur if the following condition is satisfied during timeperiod (T_(1,out)):RSRP_(measured)<RSRP_(min)−Ψ₁  (1)

Similarly, the in coverage would occur when the following condition issatisfied during certain time period (T_(1,in)):RSRP_(measured)<RSRP_(min)−Ψ₂  (2)Detection Based on Combined Common and Dedicated Channels

This procedure is applicable in case measurements are jointly based ondedicated and common channels for defining the out of coverageindication criteria. The basic principle is that in this modeout-of-coverage and in-coverage reporting is based on the downlinkchannel quality, which in turn is estimated on both common and dedicatedpilots or on reference signals.

The dedicated pilot may also represent a power control command sent onthe downlink by the base station to run uplink power control. The mainpoint is that dedicated pilot means any sequence of signals known apriori to the receiver.

Both out-of-coverage and in-coverage schemes are given below:

Out-of-Coverage Schemes:

Following two out-of-coverage reporting schemes are proposed indedicated mode:

Scheme 1 (Step-Wise):

γ=DL quality estimated (e.g. SINR or signal strength) by UE on dedicatedreference signal.

ϕ=DL quality estimated (e.g. SINR or signal strength) by UE on commonreference signal.

Start:

IF (γ<γ_(out)) over period T_(2,out)

IF (ϕ<ϕ_(out)) AND (γ<γ_(out)) over period T_(3,out)

-   -   UE reports out-of-coverage

ELSE

-   -   Go to Start.        Scheme 2 (Combined):

β=Relative DL channel quality based on common and dedicated referencesignals e.g. β could be the ratio of received power on common referencesignal to dedicated reference signal.

Start:

IF (β<β_(out)) over period T_(4,out)

UE reports out-of-coverage

ELSE

Go to Start.

Methods to Report Out-of-Coverage

Reporting on RACH

As mentioned earlier that in out of coverage situation the downlinkquality is considerably bad that could prevent the network fromassigning any scheduled resources to the UE for uplink transmission.Thus, one possibility is that UE utilizes RACH channel to report theexplicit out of coverage to the network. The following method could beused on RACH channel:

-   -   By sending unique sequence pattern

Indication by unique sequence pattern on RACH (shown in FIG. 3)

A UE will report out of coverage to the network by sending a unique(i.e. UE specific) pattern of the signature sequences on RACH channel inthe same slots where normal RACH is transmitted. The pattern shall becomprised of more than one signature sequences, which could betransmitted either in consecutive RACH time slots or each sequence inthe pattern could be transmitted in every N^(th) RACH time slot as shownin FIG. 2. In case multiple RACH slots are defined over frequency thetransmission of the out-of-coverage report may also utilize this degreeof freedom. That means transmission of the pattern would be slowed incertain time-frequency resource.

Two or more sequences in a pattern may be the same. Secondly the patterncould either use the same signature sequences as used for the normalRACH transmission or they could be reserved for out of coveragereporting. The preferred solution is to be able to use all sequences forboth normal RACH transmission and out of coverage reporting.

Let there be K unique signature sequences and M sequence per pattern.Then the total number of available unique pattern would be given byK^(M). In case K=64, M=4, the total number of unique patterns would be2²⁴, which is sufficient for all the UE operating in a large coveragearea.

Derivation of Pattern:

The UE shall derive its unique pattern by a suitable well specifiedhashing function. As an example the hashing of the UE identifier (e.g.IMSI or TMSI) could map the UE identity on one of the pattern. In thisway the UE does not have to explicitly report its UE identity. In factby decoding the pattern, the network shall be able to uniquely identifythe UE that has reported the out of coverage.

Another possibility is that network explicitly signals the index of thepattern to the UE at the time of cell selection. The index is updatedwhenever UE reselects a new cell or after handover.

When downlink coverage improves the UE reports the in coverage in anormal way that is on a shared channel following the normal resourceallocation procedure.

In order to avoid that the network considers a UE that is in coverageand performs regular RACH and by chance selects for its subsequenttransmission attempts a valid out-of-coverage pattern the networkcombines multiple information it has about a UE before declaring a UEout-of-coverage. Such information could among other include recentlyreported RRM measurements.

Another possibility is to restrict certain patterns of signaturesequences to the purpose of out-of-coverage reporting. These patternsmust not be used for regular RACH attempts by any UE.

Proactive Assignment of Resources

The UE could still report out of coverage on uplink shared channel usinghigher layer signaling (e.g. RRC) provided the network proactivelyassign the resources to the UE. The network proactively assigns theresources for uplink transmission when it notices that downlink coverageas indicated by one or more UE measurement reports, is significantlypoor such as lowest reportable SINR, RSRP etc.

The UE will then be able to report the out of coverage using theproactively assigned resources without the need for requesting any newuplink scheduling grants.

Using the above assigned resources UE either reports the out of coveragein any of the following way:

-   -   Using RRC (layer-3) message or    -   As part of the normal measurement report using out of coverage        indicator

In the latter method an extra reported value beyond the normalmeasurement reporting range is to be specified.

RACH Unique Pattern for Other Applications

The idea of sending a unique pattern using RACH is also used forreporting any other critical information in situation when UE does nothave or could not get UL scheduling grant. Thus, in use of this method,the radio base station signals a plurality of patterns of signaturesequences to a user equipment, each pattern of signature sequences beingassociated with a respective critical condition relating to operation ofthe user equipment. The user equipment receives the signalled patternsof signature sequences and then, on detection of one of the criticalconditions, the user equipment transmits the relevant pattern ofsignature sequences on the RACH. The radio base station then receivesone of the patterns of signature sequences from the user equipment onthe random access channel (RACH); and determines that the user equipmentis in the critical condition associated with the received pattern. Theradio base station can then take appropriate action. Examples ofcritical conditions are: UE running out of battery, subscriber inemergency situation, etc.

In the former situation, the network can move the UE from continuousreception mode to the discontinuous reception mode, which issignificantly more power efficient. In this way, the UE can extend itsbattery life and stay active longer. In case of repeated reporting ofthis critical condition, the network can progressively extend the DRXcycle unless maximum DRX is reached. In a E-UTRAN system, the DRX cyclein RRC connected mode (i.e. the active mode when the UE can receivedata) is between 2 ms and 2560 ms.

In the situation where the subscriber is in an emergency situation, theunique pattern can be sent in response to the subscriber pressing aspecial key on the UE. The lower layer which receives the unique patternat the base station will report the emergency situation to the higherlayers and eventually to the application layer, which will actaccordingly.

The invention claimed is:
 1. A method in a radio base station oftransmitting one or more common channels, said radio base stationcomprises means for receiving and detecting a random access channel,means for receiving measurement reports and means for assigning radioresources or scheduling grant to a user equipment, wherein the methodcomprises the steps of: signalling a unique pattern of more than onesignature sequences, to be used for reporting out of coverage, to theuser equipment (UE); and receiving the unique pattern of more than onesignature sequences on said random access channel (RACH), from the UEwhen the UE is reporting out of coverage; and interpreting the receivedunique pattern of more than one signature sequences to determine thatsaid user equipment is out of coverage.
 2. The method according to claim1, wherein the serving cell signals the unique pattern to the UE viasystem information on a broadcast channel (BCH).
 3. The method accordingto claim 1, wherein the unique pattern is signalled to the UE via ashared channel on a UE specific channel.
 4. The method according toclaim 1, wherein by receiving the unique pattern of more than onesignature sequences on RACH, the radio base station identifies the userequipment that is out of coverage.
 5. The method according to claim 4,wherein the radio base station utilizes additional available UEmeasurement reports to verify the out-of- coverage situations of theuser equipment.
 6. The method according to claim 1, wherein the uniquepattern of more than one signature sequences are only allowed for out ofcoverage reporting.
 7. The method according to claim 1, wherein theunique pattern uses signature sequences, which are shared among normalRACH transmission and out of coverage reporting.
 8. A radio base stationfor transmitting one or more common channels, comprising means forreceiving and detecting a random access channel, means for receivingmeasurement reports and means for assigning radio resources orscheduling grant to a user equipment, wherein the radio base stationfurther comprises: means for signalling a unique pattern of than onesignature sequences, to be used for reporting out of coverage, to theuser equipment (UE); means for receiving the unique pattern of more thanone signature sequences on said random access channel (RACH), from theUE when the UE is reporting out of coverage; and means for interpretingthe received unique pattern of more than one signature sequences todetermine that said user equipment is out of coverage.
 9. A method, foruse in a radio base station, wherein the method comprises the steps of:signalling a unique pattern of more than one signature sequences, to beused for reporting out of coverage, to a user equipment; receiving theunique pattern of more than one signature sequences from the userequipment on a random access channel (RACH), from the UE when the UE isreporting out of coverage; and determining in response to the receivedunique pattern of more than one signature sequences that the userequipment is out of coverage.
 10. The method according to claim 9,comprising signalling the unique pattern to the UE via systeminformation on a broadcast channel (BCH).
 11. The method according toclaim 9, comprising signalling the unique pattern to the UE via a sharedchannel on a UE specific channel.
 12. The method according to claim 9,wherein by receiving the unique pattern of more than one signaturesequences on the RACH, the radio base station identifies the userequipment that is out of coverage.
 13. The method according to claim 12,wherein the radio base station utilizes additional available UEmeasurement reports to verify the out-of- coverage situations of theuser equipment.
 14. The method according to claim 9, wherein the uniquepattern of more than one signature sequences are only allowed for out ofcoverage reporting.
 15. The method according to claim 9, wherein theunique pattern uses signature sequences, which are shared among normalRACH transmission and out of coverage reporting.
 16. A radio basestation, being adapted to: signal a unique pattern of more than onesignature sequences, to be used for reporting out of coverage, to a userequipment; receive the unique pattern of more than one signaturesequences from the user equipment on a random access channel (RACH),from the UE when the UE is reporting out of coverage; and determine inresponse to the received unique pattern of more than one signaturesequences that the user equipment is out of coverage.
 17. A method, foruse in a radio base station, wherein the method comprises the steps of:signalling a plurality of unique patterns of more than one signaturesequences to a user equipment, each unique pattern of more than onesignature sequences representing a specific critical condition relatingto operation of the user equipment (UE); receiving one of the uniquepatterns of more than one signature sequences from the user equipment ona random access channel (RACH) from the UE when the UE is in thespecific critical condition; and determining, in response to thereceiving the one of the unique patterns of more than one signaturesequences, that the user equipment is in the specific critical conditionassociated with the received unique pattern.
 18. The method as claimedin claim 17, wherein the step of signalling the plurality of uniquepatterns of more than one signature sequences to the user equipment,comprises signalling a plurality of indices that can be used by the userequipment to derive the respective signature sequences.
 19. A radio basestation, being adapted to: signal a plurality of unique patterns of morethan one signature sequences to a user equipment, each unique pattern ofmore than one signature sequence representing a specific criticalcondition relating to operation of the user equipment (UE); receive oneof the unique patterns of more than one signature sequences from theuser equipment on a random access channel (RACH) from the UE when the UEis in the specific critical condition; and determine, in response to thereceived unique pattern of more than one signature sequences, that theuser equipment is in the specific critical condition associated with thereceived unique pattern.
 20. The radio base station as claimed in claim19, being adapted to signal the plurality of unique patterns of morethan one signature sequences to the user equipment by signalling aplurality of indices that can be used by the user equipment to derivethe respective more than one signature sequences.
 21. A method, for usein a user equipment, wherein the method comprises the steps of:receiving from a radio base station a plurality of unique patterns ofmore than one signature sequences, each unique pattern of more than onesignature sequences representing a specific critical condition relatingto operation of the user equipment; determining that one of saidspecific critical conditions applies; and transmitting one of the uniquepatterns of more than one signature sequences, associated with thespecific critical condition, on a random access channel (RACH).
 22. Themethod as claimed in claim 21, wherein the step of receiving from theradio base station the plurality of unique patterns of more than onesignature sequences comprises receiving a plurality of indices and usingthe received indices to derive the respective more than one signaturesequences.
 23. A user equipment, adapted to: receive from a radio basestation a plurality of unique patterns of more than one signaturesequences, each unique pattern of more than one signature sequencesrepresenting a specific critical condition relating to operation of theuser equipment; determine that one of said specific critical conditionsapplies; and transmit one of the unique patterns of more than onesignature sequences, associated with the specific critical condition, ona random access channel (RACH).
 24. The user equipment as claimed inclaim 23, adapted to receive from the radio base station the pluralityof unique patterns of more than one signature sequences by receiving aplurality of indices and using the received indices to derive therespective more than one signature sequences.